Heat transfer method for solvent recovery and aromatic crystallization



1960 M. R. FENSKE ETAL 2,920,120

HEAT TRANSFER METHOD FOR sowmw RECOVERY AND AROMATIC CRYSTALLIZATIONFiled Jan. 23, 1957 3 Sheets-Sheet 1 P Lu 1| 6 m 2 D 4. i o (D I 0 a r nm 0 6 (O T 4 E D 9 u.

J, f v

f n to Merrell R. Fenske I ventors Walter G. Braun By 1 1%? H-MKAflorney7 Jan. 5, 1960 F s ET AL 2,920,120

HEAT TRANSFER METHOD FOR SOLVENT RECOVERY AND AROMATIC CRYSTALLIZATIONFiled Jan. 23. 1957 3 Sheets-Sheet 2 209 FIGURE-2 309 FIGURE-3 401 O 50/J 4I5 l -4o2 L 4l4 /-K(--D 5n v 512 FIGURE 4 509m FIGURE 5 Merrell R.Fenske Walter 6. Braun Inventors By E/C Attorney 960 M. R. FENSKE ETAL2,920,120

- HEAT TRANSFER METHOD FOR SOLVENT RECOVERY AND AROMATICCRYSTALLIZAT'ION Filed Jan. 23, 1957 3 Sheets-Sheet 3 CRYSTALS y Merrel!R. Fenske Walter G. Braun '"Venfors Y (E/C- H Attorney HEAT TRANSFERMETHOD FOR SOLVENT RE- COVERY AND AROMATIC RYSTALLIZATION Pa., assignorsto Esso Research and Engineering Company, a corporation of DelawareApplication January 23, 1957, Serial No. 635,868 20 Claims. c1. 26 -674)This invention relates to an improved method of cooling a liquidsolution containing a substantial portion of a;volatile solvent. Moreparticularly, the invention relates to the transfer of heat from oneliquid stream to another liquid stream using a circulating inert gas. Ina specific embodiment, the invention relates to the liquid ammoniaextraction of durene range hydrocarbons from a catalytic naphtha whereincooling of the extract phase Merrell R. Fenske and Walter G..Braun,State College,

for solvent recovery and durene crystallization as well as the extractortemperature control is efiected by circulationof an inert gas such asnitrogen therethrough.

Cooling of a solution to precipitate a solute is a com-. 7

surfaces which can foul or on which the solute can pre-- cipitate. Arelatively large temperature difierence must be maintained between thetwo fluid streams to transfer heat because of the heat resistances ofthe wall and the fluid films.

When 'one of the fluids is a gas and the other a liquid the wall isoften not necessary, particularly when there is no problem of mutualcontamination, e.g., in water cooling towers. Temperaturedifierencesbetween the two streams at any given point in such a direct-contactapparatus can be less'than if the streams were separated by a wall. Ifthe liquid stream is non-volatile the heat interchanged with the gas isessentially sensible heat. If the liquid stream is volatile there may bean exchange of both sensible heat and also latent heat because ofevaporation or condensation of some of the volatile liquid.

It is an object of'this invention to provide an improved method of heatexchange using direct contact between a liquid and a gas. It is afurther object to provide a method of transferring heat from one liquidstream to another by employing direct contact between one or morecirculating inert gas streams and each of the liquid streams. It is alsoan object to provide a heat exchange method between two fluid streams inwhich a minimum of energy is spent and comparatively little of the heat'the invention.

Figure 7 ;is:,a schematic flow sheet of a process em-,bodyingzthepresent invention.

Other objects will become apparent 2,926,120 Patented Jan. 5, 1960 Inaccordance with the present invention, heat transfer between two liquidstreams is effected by contacting a relatively warm volatile liquidcountercurrent to a stream of a dryand cool inert gas, whereby some ofthe liquid evaporates into the gas phase and the liquid is cooled. Theextent of the cooling is dependent on the amount of liquid evaporatedand on the heat of vaporization. .The amount of liquid evaporated is, inturn, dependent on the vapor pressure-temperature relationship of theliquid and on the total pressure of the system. The gas phase is warmedand picks up more volatilized liquid as it passes through thecountercurrentcontacting unit.

In a companion step, a relatively cold stream of the volatile liquidflows countercurrent to the warm inert gas which was essentiallysaturated with vapors of the volatile liquid in the previous step. As aresult, the vapors will condense out of the gas phase and into thecountercurrent flowing, relatively cool liquid phase. The heat ofvaporization thusreleased will tend to warm the liquid phase and therebyin effect transfer heat thereto from the first mentioned, relativelywarm liquid stream. The gas phase is cooled and denuded of its vaporizedliquid content as it flows through the countercurrent contacting unit,and

fmay eventually be recycled to the first step of the process.

Figure 1 illustrates an apparatus in which a volatile, liquid .can becooled to some low temperature in order,

for example, to precipitate asolute, and-then be reheated. Referring-toFigure 1, a warm volatile liquid 1, e.g.,asolventextract containing anormally solid solute, enters the top of a vapor liquid contactor 2 suchas a packed or a packed or a bubble-cap tower operating in essentiallyan adiabatic manner. Passing countercurrently to this liquid stream -.isa relativelycool, dry stream of an inert gas which is continuouslyrecirculated through the contactor by means of-blower 3. Some of thevolatile liquid evaporates into the circulating gas stream as it passesdown through the upper part of ,the contactor and the liquid stream isthereby cooled while the gas is warmed and humidified by the vaporizedliquid. Approximately in the center of the contactor a partition isplaced which traps the liquid passing down the unit but allows the freeupward passage of vapor. The partition as illustrated in the diagramconsists essentially of a plate or diaphragm 6 into which verticallengths of-pipe 7 have been fixed. Small umbrellas 8 cover each of thetubes to prevent liquid from dropping through them. As cold liquidcollects on the plate and around tube 7 it is removed via line 9 to apump 10 and then to a heat exchanger 11 where it can be further cooledto the desired precipitation temperature. In the example chosen, 12represents a vessel, e.g., a basket-type centrifuge, wherein the solutecan precipitate and from which it is removed at 13. The cold liquidsolvent, denuded of its solute, then returns via line 14 to thecontactor where it is introduced, just below the liquid separator plate6. The cold liquid stream then flows countercurrently to the warm andhumid gas stream and is thereby reheated while the gas is cooled anddried. The liquid, reheated and freed of solute, leaves the contactorvia line 15 and may be reused, for instance, in the principal extractionstage, not shown.

The gas stream, warm and essentially saturated with volatilized liquid,leaves the top of the contactor by line 5 and is returned by means ofblower 3 and line 4 to the bottom where it meets cold descending liquidwhich cools it and condenses out much of the volatilized liquid, asalready described. The dry gas then passes from the lower to the uppersection of the contactor, through the vapor-liquid separator 6, at thesame, or at a slightly higher temperature than the liquid streamentering via- 14. In passing upward through the upper section of the istransferred through an impermeable wall in heat exchanger 11. As will beillustrated later, a liquid stream such as liquid ammonia can be cooledfrom 60 F. to 20 F. in this manner. Figure 1 and the modifications shownin Figures 2, 3, 4, 5 and 6 illustrate the vaporliquid contactor for theliquid cooling and liquid heating operations as a single unit. However,it is obvious that the heating and cooling operations could be effectedin separate units suitably-connected by vapor and liquid lines.

It is an important requirement of this invention that.

in the upper or liquid cooling section of the contactor the temperatureof the liquid' be above the saturation temperature of the gas stream atany point in order for liquid to evaporate. In the bottom section of thecontactor the liquid temperature must be below the saturationtemperature of the gas stream at any point in order for liquid tocondense out of the gas. To accomplish this, it is necessary to reducethe enthalpy of either the liquid stream as it passes from the liquidcooling section to the liquid heating section, or the gas as it passesfrom The enthalpy of the liquid stream can be reduced by cooling. Theenthalpy of the gas stream can be reduced by cooling or by reducing theconcentration of vaporized the liquid heating section to the liquidcooling section."

liquid in the gas, or by'compression and expansion, or-

by any suitable combination of the foregoing expedients. In thearrangement shown in Figure l, exchangerll cools the liquid, as itpasses from the upper to the lower section of the apparatus, to atemperature below the saturation temperature of the gas stream at thatlower section of the contactor. The arrangements shown in Figures 2-5employ some of the other methods of meeting the above-mentionedrequirement.

Referring to Figure 2, 'a warm volatile liquid extract 201 comes intothe top of a vapor-liquid contactor 202 and flows downwardcountercurrently to a gas stream. The contactor is divided into twosections by means of an impermeable plate 203, which prevents thepassage of liquid or vapor. The liquid is cooled in the upper section ofthe contactor and is sent via line 204 through pump 205 directly to avessel 206 where a precipitated solute can be removed via 207 and thecold solvent returned via line 208 to the lower section of thecontactor. The liquid is reheated by the circulating gas in this lowersection and leaves via 209. The warm inert gas containing a largeportion of vaporized liquid leaves the top of the contactor via line 210and is recirculated into the bottom by means of blower 211 and inletline 212. The gas stream leaves the lower section of the contactor vialine 213, is cooled in exchanger 214, and returns to the contactor justabove the dividing plate 203 via line 215. Thus the enthalpy of the gasstream is reduced as it passes from the lower to the upper section ofthe contactor. Some liquid is condensed in exchanger 214 and may bereturned to the contactor via line 216.

A third method of operation is illustrated in Figure 3. It isessentially similar to that described previously with reference toFigure 2 except that instead of refrigeration of the gas stream betweenthe upper and lower sections of the contactor, the gas leaving thecontactor via 313 is at least partially denuded of its volatilizedsolvent in an appropriate unit 314 by a process such as water scrubbing,adsorption on charcoal, or similar processes such as chemical removalthat will be referred to herein by the generic expression physicalabsorption. The dried or essentially pure inert gas is returned to thecontactor via "pressure than the upper section.

4 315 and the recovered volatile liquid removed via line 316.

A fourth method of operation is illustrated in Figure 4 wherein theliquid extract 401 to be cooled enters the contactor 402 and flowsdownward countercurrently to a circulating gas stream., In approximatelythe middle of the tower the cooled liquid is collected on plate 404 andremoved via line 407. The liquid is pumped by pump 408 directly to asettling vessel 409 where the precipitated solute can be removed as 410.The collecting plate 404, vertical pipes 405 and umbrellas 406 have beenpreviously described with reference to Figure l. The denuded solvent isreturned to the contactor via 411 and is reheated and leaves as liquidstream 412. The gas stream passes through the contactor countercurrentlyto the liquid phase, entering the bottom as stream 413. Immediatelyabove the gas-liquid separator plate 404 a stream of dry inert gasenters via line 414 and is added to the gas stream in the contactor. Thegas leaving the top of the contactor is split into two streams, onebeing recirculated to the gas dehumidifying or bottom portion of thecontactor by blower 403, and the other leaving via line 415, is denudedof its vaporized liquid by some process such as gas washing oradsorption in zone 416 and returned as stream 414, The purpose of addinga dry inert gas streamin the upper or cooling section of the contactoris to reduce the vaporized liquid concentration, and thus the enthalpy,of the gas stream so that evaporation, and hence cooling, can takeeffect.

Still another method of operation of the heat exchanger is illustratedin Figure 5. Warm volatile liquid as stream "501 enters the top ofcontacts 502 and flows downward counterourrently to a circulating gasstream. The contactor is divided into two sections by plate 503 whichprevents the passage of liquid or vapor and allows the lower section ofthe contactor to operate at a higher The liquid having been cooled inthe upper section of the contactor is removed via line 504 through pump505 to a vessel 506 where a precipitated solute can be removed at 507.The cold liquid phase returns to the lower section of the contactor vialine 508, flows downward countercurrent to the gas stream, and leaves asstream 509. The gas stream leaves the top of the contactor via line 510,is compressed by compressor 511, e.g., to a pressure between about 175and 250 p.s.i.a., and recirculated to the bottom of the contactor vialine 512. Immediately below the dividing plate 503, which separates theupper and lower sections of the contactor, the cold gas stream isremoved by line 513 and allowed to expand through an expansion enginesuch as a turbine 514 to a lower pressure, e.g., between about 100 and150 p.s.i.a. In other words, the required enthalpy offset is provided byoperating the lower section of the contactor at a high pressure, eg 200p.s.i.a., while operating the upper section at a lower pressure, e.g.125 p.s.i.a. The gas in then passed by line 515 into the contactorimmediately above the dividing plate 503 and passed upwardcountercurrently to the descending warm liquid. The expansion isaccomplished through the expansion engine 514 in order to reduce theenthalpy of the gas stream 7 and at the same time produce work to ofisetsome of the work required by compressor 511.

Figures 1 to illustrate modifications suitable for maintaining thecirculating gas stream or the liquid stream properties such that liquidwill evaporate into the gas stream in the liquid cooling section of theapparatus and such that liquid will condense from the circulating gasstream into theliquid stream and heat In Figure 1 the liquid streamenthalpy is reduced in exchanger 11 while in Figure 2 the gas streamenthalpy is reduced by cooling in exchanger 214. In Figures 3 and 4 thegas stream enthalpy per unit weight of inert gas is reduced, in theformer case by reduction of vaporized liquid content of the gas, and inthe latter case by dilurise or the gas st eam with r jessent'iallydryinertga's. In Figure .5, the gas stream-en lpy is reduced by expansionthrough an engine. ,Theenthalpy of the liquid referred to here istakenin the sense as the heat content per unitweight of liquidrelativ eto achosen reference temperature. The enthalpy of the gas, in the sense usedhere, is the enthalpy per unit weight of inert gas relative to a chosenreference temperature plus the enthalpy of the weight of vaporizedliquid associated with the unit weight of inert gas';

A certain quantity of energy 'is required to maintain "the necessaryenthalpies of the liquid orgas streams such that evaporation andcondensation will take place. This energy is supplied either to cool theliquid or gas streams (Figures 1 and 2), to reduce the vaporized liquidcontent of the gas stream (Figuresd and 4), or to drive the compressorover and, above the energy that is recovered by the expansion engine(Figure 5).

In order to reduce the energy requirement it has been found advantageousto maintain a higher ratio of'gas flow to liquid flow at the coldsection of the, contactor, i.e., the middle section extending from apoint just above the cold liquid withdrawal line to a point just beloWthe cold liquid return line, than at the warmer sections, i.e.,the'sections near the top of the liquid cooling zone and near the bottomof the liquid heating Zone.

Thus, the enthalpy of the gas stream is maintained relatively close. toits equilibrium enthalpy throughout the ..,L.. I The temperaturesprevailing in the contac'tors of Figures 1 through depend? uponthetemperature of the entering liquid and the temperature to which thisliquid is to belcooled". In general, thesetemperatures lie belength ofthe contactor, andthe enthalpy requirement to produce the enthalpyoffset between the upper and lower cold section of the contactor isillustrated in Figure 6.

The basic arrangement of the liquid flow has'been previously describedWith Figure 1. The gas, however, is

circulated in two or more streams. As shown in Figure 6, the primary gasstream leaves the top of the contactor via line 610 and is recirculatedto the bottom via line 612 by means of blower 611; A secondary gasstream leaves the liquid cooling section of thecontactor via line 613 atsome point between the primary gas stream take-01f 610 and the point ofcold liquid withdrawal 603. This secondary gas stream is recirculated bymeans of blower 614 and return via line 615 at a point between the coldliquid return line 608' and the primary recirculating gas inlet 612which is approximately at the same temperature level as the aforesaidpoint of secondary gas withdrawal. The ratio of gas 'in the" secondarygas stream to that in the primary stream is maintained between aboutl to1 and 3 to 1.

For cooling liquids to very low temperatures and where a minimum ofenergy is to be expended it may be advantageous to use additionaloirculatin'g'ga's streams.

skilled in the art simply by maintaining a liquid to gas ratio at allpoints in the contactor so that the enthalpy of the gas and itsequilibrium enthalpy differ just enough to "allow the transfer of heatto take place.

In the same manner that Figure 6 is a variation of Figure l, the use ofmultiple recirculatinggas streams can also be applied. to thearrangements illustrated in Figures'Z, 3, 4 and 5 in order to reduce theenergy requirements.

The general principle regarding the total pressure on the contactorsdescribed in Figures 1 through 6 is that the pressure is composed of thepartial pressure ofthe liquid phase plus the partial pressure of theinert gas. Accordiiigly, the total pressure on the contractor is thepartial on the liquid flow rate. These can be calculated by thosepressure of the solvent at the warmest point plus at least and severalmodifications tween about '40 F; and about +300 F.

Inconnection with-Figures- 1 through 6, when liquid ammonia comprisesthe bulk of the liquid phase and when the liquid phase enters at about60 to F. and is cooled to about ,-4,0 to '+60'T" F., pressures in therange of to 400 p.s.i. have been found suitable.

c Having described. the basic principles of our invention ere'of, theinvention will be further illustrated with referents-m Figure 7 whereindurene is extracted from" catalytic naphthas. Dure'ne,1,2,4,5-tetramethyl benze becoming an increasingly importantpetrochemical at finds uses in the synthesis ofQfiber-fo'nningmenus-,ae; Much of the description herein will concern the extraction ofdurene with a liquid ammonia solvent using nitrogen as" the inert gas inthe heat transfer operation's, bi'1t it should be understood that thisdescription is being given' primarily-for purposes of illustrationrather than limitation. It is to be under- I Figure 7 shows alilarifliior recovering durene in accordance with the. invention: oneinventive feature relates to continuously cycling a'stream of nitrogenthrough some extractor stages to fevaporate'" some of the solvent andreduce the temperature;'and their contacting the nitrogen streamcountercurre'ntly with cold solvent to reduce the solvent vapor contentoffthe nitrogen. ,Anotherffeature relates to cycling another portion ofinert' gas to cool the extract phasev of theextraction, by evaporatingsome of the liquid therefrom, for purposes of solvent recovery anddurene crystallization. [The remaining op-' erations of the process areessentially conventional, but the over-all combination provides animproved, economical process of extraction; The most conservativemeasures are taken to lower external energy and heat-transferrequirements.

Durene is one of the aromatic hydrocarbons found in catalyticallycracked. naphthas and 'it'can be concentrated to 2 to 15 percent bycareful fractional distillation. Preferred feed stocks may have aboilingrange from 370 to 400 F.,' though wider or narrower cuts may beused;- Referring to Figure 7, the naphtha feed 702 enters on a stagenear the middle of liquid extractor 701 which is operated at a pressurebetweenabo'ut 150 and 400 p.s.i.a. A solvent containing 60 to"1 00'%ammonia is introduced as a liquid through line 703. Bo'thtli'e' naphthaand the enter evaporating drum 705. The ratio of solvent to feed ispreferably maintained between about2.5 to 1 and 5 to 1. The extractorshould have nine to twenty theoretical stages.

Maintenance of the proper hydrocarbon solubility in the extract phase inthe extractor is important, particularly at the extract or stripping endwhere the concentration of aromatics is high. The reduction of thetemperature in the upper or stripping stages is one method ofmaintaining the desired solubility. An important phase of this inventionis the transfer of heat from the liquid on these stages to a coldsolvent stream by means of a circulating nitrogen gas stream. Thisnitrogen together with some solvent vapor enters these extractionstages, that are to be cooled, at a temperature 5 to 20 F. cooler thanextract stream 704, through lines 707, 708,

.704, containing from 10 to 30% hydrocarbons, is about 30'to 50 F.cooler than feed stream 702. The nitrogen, containing from 0.3 to poundsof ammonia per pound of nitrogen, leaves the extractor via lines 711,712,

713 and 714 to vapor-liquid contactor 718 which serves as a solventheater. This may be a packed-or bubble cap tower having eight to tenvapor-liquid contact stages and operating in an essentially adiabaticmanner. Within the vapor-liquid contactor the gas stream flowscountercurrently to a descending stream of cold solvent, at atemperature between about 30 to 80 F., entering by line 719. Solventvapors associated with the inert gas condense into the coldliquid andheat the liquid to about 80 to 130 F. Thewarmed liquid solvent leavesvia line 720, and the nitrogen'stream, now containing from 0.1 to 2 lbs.NH /lb. N leaves via line 721 at a temperature between about 50 and 90F. to blower 722 which recirculates the gas. Makeup gas may beintroduced through line 766.

Within the evaporating drum 705, operated at a pressure between about 50and 200 p.s.i.a., sufiicient selfevaporation of the solvent occurs tocool the extract phase about another to 30 F. to a temperature be tweenabout 40 and 80 F., thus precipitating some of the dissolvedhydrocarbons. These hydrocarbons are returned through line 715 to thetop of the extractor to comprise about 80 to 99% of the reflux. Solventvapors generated in drum 705 are passed via line 716 into compressor 723and after compression are condensed in 725 and returned to the mainsolvent stream entering the extractor. The remaining extract layer indrum 705 leaves through line 717 and enters the top of the liquidcooling section of the direct contact exchanger 726, which operates at apressure of about 150 to 400 p.s.i.a. Contactor 726 is a secondimportant phase of the embodiment and is a direct application of theinvention illustrated in Figures 1 and 6. It has a total of tovapor-liquid contact stages, about half in the liquid heating sectionand half in the liquid cooling section.

In the vapor-liquid contactor 726 the extract phase 717 is cooled about60 to 100 F., e.g. from 60 F. to 20 F., by countercurrent contact with acirculating stream of nitrogen and solvent vapor. The nitrogen andsolvent vapor are recirculated in lines 763 and 764 by means of blowers744 and 745 as described previously with reference to Figure 6. Makeupnitrogen may be introduced through line 765. The cold liquid streamleaves to cooling section of the contactor via line 727 and proceeds toa self-evaporation chiller 728, which operates at about atmosphericpressure, Where some of the solvent is evaporated, reducing the liquidtemperature about another 5 to 30 F., e.g. to between 30 and ,50 F. Thenon-vapor material 728B in chiller 728 is a slurrycontaining about 5 tohydrocarbons and consisting of three phases: a liquid hydrocarbon phasewhich is essentially aromatic in nature, a liquid solvent phasecontaining about 2% dissolved hydrocarbon, and a solid crystalline phaseconsisting largely of durene. The slurry is sent via line 730 to afilter, centrifuge or other mechanical separation device 731 where thesolid durene crystals 732 are removed. The filtrate is removed-vialine733 to a settling drum 734 where a solvent-rich layer 734A and anextract hydrocarbon layer 734B are formed. The solvent-rich layer, 5 toF. cooler than exit stream 727, is sent via line 736 to the liquidheating section of the contactor 726 where it descends countercurrentlyto the nitrogen-solvent vapor stream and is warmed about 75 to 115 F.,e.g. from 40 to 45 F. The ratio of liquid solvent to nitrogen-solventgas is maintained between about 5 to 1 and 8 to 1 in the warmer upperand lower extremity sections of the contactor and between aboutl to 1and 3 to 1 in the colder middle sections for reasons set forthhereinbefore with reference to Figure 6.

The solvent removed by line 739 is sufiiciently pure, i.e., containingless than 2% hydrocarbon, when mixed with the other solvent streams fromcompressor 740 and distillation column 755 to be returned to extractor701 via line 703. The hydrocarbon-rich phase 734B is removed from drum734 via line 735, is passed through an indirect heat exchange coil 737in contactor 726, and is sent via line 738 to water washing tower 747. 5

Drum 728 is autorefrigerated by evaporation of solvent from the solventextract phase 728A. The solvent vapors pass through line 729 tocompressors 740 and 741 in a proportion between about 1 to 2 and 2 to 1.The compressed solvent vapor 742 from compressor 740 is combined withthe cold liquid solvent stream 739 from contactor 726, therebycondensed, and introduced into solvent heater 718 via line 719 andfinally returned to extractor 701 as aforesaid. The other part of thesolvent vapor, compressed in compressor 741, is condensed in condenser725 and returned to the solvent stream 703 entering the extractor.

Towers 746 and 747 operate identically to remove solvent from therafiinate and extract phases by water washing. Water at a temperaturebetween about 50 and 200 F. and at about atmospheric pressure enters thetop of both towers through line 748 and flows downward with the.hydrocarbons, removing any remaining solvent from the hydrocarbons bydissolving them in the water. Two liquid phases collect in each of thetwo settling drums 749 and 750 at the base of towers 746 and 747. Thehydrocarbon products 7 49A and 750A are removed, at temperatures betweenabout 50 and 200 F. and at about atmospheric pressures, as raffinateproduct 751 and extract product 752. The solvent-containing water 7493and 750B is removed from each drum via 7.53, passed through heatexchanger 754, and entered'into distillationcolumn 755'.v This column755 operatescom ventionally at a pressure between about 200 and 400p.s.i.a. with a reboiler 756 at the base of the column containing aheating coil 757 wherein steam or other suitable heating fluid flows.The column is provided with a condenser 758 and some method ofproportioning reflux 759 to the column. Solvent distillate is added vialine 760 to the solvent stream 703 entering the extractor. The liquidwater at about 300 to 500 F. and 200 to 400 p.s.i..a. leaving thereboiler via line 761 is cooled in heat exchangers 754 and 762; thiswater enters via line 748, and flows to the water scrubbing towers 746and 747.

The invention with respect to the extraction of durene fromheavycatalytic naphthas will now be described more specifically in thefollowing illustrative embodiment.

EXAMPLE Using a process identical to that described with reference toFigure 7, the following data shown in Tables 1 and2 illustrate how anyield of durene crystals can be obtained from a heavy catalytic naphtha,containing 11% durene and boiling from 370 to 400 F., using as a solventa mixture of percent ammonia and '10 percent monomethylamine, with asolvent to feed ratio of 5 ml. The various streams are identified in thetables by symbols corresponding to the respective v numerals appearingin Figure 7.

quired 425 HP. The coolingwater requirement'sand the number. of indirectheat exchangers, were. also cane is'pondingly greater. In general, "themethod of indirect hea 'axchan e herein disclosed provides anattractive, economical operation when applied to a liquid extractionprocess. The invention may be applied, however, not ny to. naphthaextraction, but also to such processes as dewaxing to cool the oil-wax-solvent slurry and to removing water wherein water is frozen out of asolvent. It may. be'used essentially wherever it is desired to cool a vola tile liquid streamv 'at one point and, by the transfer 16 poneiit isoften the principal solvent or extractant, such as ammonia, liquidsulfur dioxide, and the like. In this case, it, is preferable to use asuitable modifying agent iri admixture with the volatile liquid; such asthe methyl amines, ethylamines, aniline, pyridine, methanol, loweralcohdls and ethers, which increase the solvent power of ammonia; orwater, ethylene glycol, ethylene diamine; forirrainide, and low meltingparaffinic hydrocarbons, whichdecrease the solvent power of ammonia.Particularly preferable in this invention is a solvent comprising 60 to100% ammonia in admixture with 40 to 0% mono- Table 1 nYDRooAnBoN ANDSOLVENT GLOW Wt. l er- Flow, 1,000s-I 7bs. Per Hour 7 cent I, Wt. Per-Temp, Press, Symbol Name Hydrocent F. p.s.1.a.

' carbon Solvent Hydro- Solvent Total carbon Fee l 100 0 5 120 300 15.50 15.5 Solvent Feed-' 5 1.2 98.8 120 300 0.9 77.5 78.4 18. 7 81.3 80 30016. 4 71.8 88. 2 95.1 5 4.9 120 300 7.8 0.4 8.2 95.1 4.9 60 300 7.8 0.48.2 0v 100 60 107 0 3.5 3.5 11.3 -88.7 60 107 8.7 67.9 76.6 1.4 98.6 55300 0.9 66.7 67.6 1.3 98.7 1 105 300 0.9 172.3 73.3 Extract Phase 12.8-87.2 175. 8.7 59.0 67.7 Solvent Vapor 0 100 10' 0 2.3 2. 3 Extract Phase13.3 87.0 40 10 8. 7 56. 7 65. 4 Durene Crystals (80% Yield) 100.0 0 4010, 1. 39 0 1. 39 Extract Phase 11.4 88.6 40 10 7. 3 56.7 64.0 ExtractProduct .98; 0 2. 0 -40 10 6. 4 0.1 6. 5 Solvent 1.6 98.4 -40 10 0.956.6 57.5 Extract 'Product 98.0 q 2.0 35 10 6.4 0.1 6.5 Solvent- 1.498.6 175 0.9 65.5 06.5 0 100 290 98 0 1.2 1.2 0 100 430 300 0 1.1 1.1 v,..0 0 100' 20 100 0 5 .100 20 7.5 o 7.8 100 0 100 20 6.4 0 6.4 0, 10020 -0 100 90 300 Solvent 0 100 90 300 0 0.5 05 Fresh Water Y, 0 O Y 417'300 Table 2 40 methylamine. An extract phase leaving the extractorINERT GAS RECYOLE containing 60 to 90% ammonia, 0 to 18%monomethylyamine, and 10 to 25% hydrocarbons is a highly prefer- Flow,able condition. Symbol g n, g 6 0 The solvent may also comprise anon-volatile princier 2 lll pal solvent-such asaniline, glycols,furfural, or phenols,

, in admixture with a minor proportion of a volatile com- 80 300 M32 7Aponent such as ammonia, propane, orone of the Freon- 38 288 g typechlorofiuoro alkanes. Multi-component solvent no I 1 mixtures containingliquid sulfur dioxide, such as sulfur 70, 300 0.457 -5 dioxide and.benzene, are also suitable. In addition, the 48 175 0.585 11.0 50 11 1750,176 23,0 volatlle component can be one of the components belngseparated, such as propane in the separation of propylene of the heatthereby lost, reheat the same liquid atanother point in the process.

Thus, in the dewaxing of oils, propane, butane, or ammonia may be usedas the volatile component in the liquid phase and the process wouldoperate essentially described, with wax crystals being produced fromfilter 731, and a propane-wax free oil solution would leave via line 739of Figure 7. The warm propane-waxy oil solution would enter tower726'via line 717 of Figure'7.

Other important petrochemical separations where this process can be usedare in the liquid extraction of naphthas containing cyclohcxane,p-xylene, naphthalene, and

styrene, or any one of them. The extraction and 0178- tallization stepsto separate these hydrocarbons would be essentially as described inFigure 7. It should be understood, however, that the crystallizationstep isf'not necessary to the operation of this invention. The processdisclosed is well suited to therecovery of solvent in a liquidextraction process by chilling, as described in US. Patent 2,728,708.

It has already been pointed out that the liquid phase to be cooled bythe present invention must comprise, at least in part, a volatilecomponent. This volatile comfrom propane, or butane in the separation ofbutylenes by liquidv extraction with. a high boiling solvent such asglycol oraniline. 1

The term inert gas is intended to mean a gas that is. non-:condensableunder the operating conditions, but may be soluble in the liquid to becooled. It is inert towards the ordinary materials of construction, andthe 'fluidingredients used. in the process. Examples are nitrogen,hydrogen, the low boiling hydrocarbons such as methane and ethane,helium, low boiling chlorofl-uoro- -methanes suchasdichlorodifiuoromethane, and the like. The term dry inert gas means thatthe non-condensable "gas has a low'content 'of the vaporized volatileliquid, that this dry gas has been produced by'reducing its content ofvolatile liquid in a preceding step.

The process can be operated in reverse. That is, insteadof starting witha warm liquid, first cooling it, and then reheating it; the same processcan be used starting "with 'a'c'ool'liquid, by first heating it toperform some desired physical or chemical change, and then cooling itback'to the approximate temperature level at which it entered theprocess.

Having thus presented a general description and il- 11 lustrativeembodiments of the present invention, the true scope is now set forth inthe appended claims.

The claimed invention is:

1. A process for transferring heat in a heat transfer system from arelatively warm liquid stream containing a substantial fraction of avolatile liquid to a second portion of said stream, which comprisespassing a dry insert gas stream countercurrently to the liquid stream ina cooling zone wherein the liquid is at a temperature above thesaturation temperature of the gas, thereby evaporating a portion of saidvolatile liquid into the gas stream and cooling the liquid, passing theresulting vaporcontaining gas stream countercurrently to the cooledliquid stream in a heating zone wherein the liquid is at a temperaturebelow the saturation temperature of the gas, thereby condensing thevapors into said liquid stream and heating the liquid, continuouslyremoving heated liquid from said system, returning the vapor-denuded gasstream to the cooling zone, and reducing the enthalpy of one of thestreams at a stage between the cooling and heating zones sufliciently tomaintain the temperature of the liquid above the saturation temperatureof the gas in said cooling zone and below the saturation temperature ofthe gas in said heating zone.

2. A method according to claim 1 wherein the enthalpy of the liquid isreduced by further cooling said liquid at a point after leaving saidcooling zone and before entering said heating zone.

3. A method according to claim 1 wherein the enthalpy of the gas streamis reduced by cooling said gas stream at a point after leaving saidheating zone and before entering said cooling zone.

4. A method according to claim 1 wherein the enthalpy of the gas streamis reduced by physical absorption of the volatile liquid vapors fromsaid gas stream at a point after leaving said heating zone and beforeentering said cooling zone.

5. A method according to claim 1 wherein the enthalpy of the inert gasstream is reduced by diluting said gas stream with an additional amountof at least partially dried inert gas at a point after leavingsaidheating zone and before entering said cooling zone.

6. A method according to claim 1 wherein the enthalpy of the gas streamis reduced by reducing the pressure of said gas stream at a point afterleaving said heating zone and before entering said cooling zone, andincreasing the pressure of said gas at a point after leaving saidcooling zone and before entering said heating zone.

7. A process for the transfer of heat which comprises passing a streamof a relatively warm liquid solution containing a solute and asubstantial fraction of a volatile liquid solvent into a liquid coolingzone, countercurrently contacting with said solution a relatively dryinert gas stream, thereby evaporating an amount of said volatile liquidinto said gas stream and cooling said liquid stream, removing the cooledliquid stream from said cooling zone, recoveringprecipitated solute fromsaid cooled liquid stream, passing said cooled, relatively solute-freeliquid stream into a heating zone, countercurrently contacting saidcooled, relatively solute-free'liquid in said heating zone with thevapor-enriched gas from said cooling zone, thereby condensing thevolatile liquid vapors from said gas into and heating said. liquidstream, recycling the resulting relatively dry gas stream to saidcooling zone, continuously recovering relatively warm liquid solvent,and reducing the enthalpy of at least one of the streams between theirwithdrawal from their respective cooling zones and their introductioninto their respective heating zones so that the temperature of theliquid is maintained above the saturation temperature of the gas in saidcooling zone and below the saturation temperature of the gas in saidheating zone.

8. A process according to claim 7 wherein said inert gas circulatesbetween said heating and cooling zones in a plurality of streams suchthat a higher gas to liquid flow ratio is maintained at the coldsections of the contacting zones than at the warmer sections. 7

9. An extraction process which comprises extracting relatively solubleconstituents from a hydrocarbon feed in a multistage liquid extractionzone with a liquid solvent containing a substantial fraction of avolatile liquid, passing a relatively dry inert gas through severalstages of the said extraction zone wherein a portion of the liquidsolvent evaporates into the inert gas stream, thereby cooling saidsolvent and humidifying said gas, withdrawing a liquid solvent extractfrom said extraction zone, cooling the withdrawn extract until at leasta part of the hydrocarbon dissolved therein is precipitated, separatingthe precipitated hydrocarbon, countercurrently contacting the cooledhydrocarbon-denuded solvent with the humidified inert gas from theextraction zone in a gas-liquid contacting zone, thereby condensing thevapors into the liquid solvent and reheating said solvent, and recyclingthe resulting relatively dry gas and reheated solvent to the extractionzone.

10. A process according to claim 9 wherein the hydro- I carbonprecipitated from'the solvent comprises both a liquid phase and anaromatic crystalline hydrocarbon phase, each phase is separated from theother and from the entrained solvent therein, and the separated solventis recycled to the extraction zone.

11."A process according to claim 9 wherein said solvent comprises liquidammonia and wherein sufficient ammonia is evaporated into the inert gasin the extraction zone to maintain theextract stream leaving saidextraction zone at a temperature 30 to 50 F. cooler than the hydrocarbonfeed stage.

12. A process according to claim 9 wherein said liquid solvent comprises60 to weight percent ammonia and 40 to 0 weight percent of amethylarnine.

13. A process according to claim 9 wherein said inert gas circulatesbetween said heating and cooling zones in a plurality of streams suchthat a higher gas to liquid flow ratio is maintained at the coldsections of the contacting zones than at the warmer sections.

14. A process which comprises extracting relatively soluble aromaticconstituents from a hydrocarbon feed in a multistage liquid extractionzone with a liquid solvent containing a substantial fraction of liquidammonia, passing a plurality of inert gas streams through a plurality ofstages of said liquid extraction zone so as to evaporate gradually ofsufiicient portion of the liquid ammonia into said inert gas streams tocool said solvent to a temperature 30 to 50 F. cooler than thehydrocarbon feed stage, withdrawing an extract stream, precipitating aportion of the hydrocarbons contained in the withdrawn extract in aseparation zone, recycling said precipitated hydrocarbons to theextraction zone as retlux, passing the remaining solvent extract layerfrom said separation zone through an extract cooling zonecountercurrently to another relatively dry inert gas stream whereby aportion of volatile solvent evaporates from said extract into the gasstream cooling said extract, passing the resulting humidified gas streamfrom said extract cooling zone into a first solvent heating zone, whereit is countercurrently contacted with a relatively cool liquid solventphase, removing the cooled extract from said cooling zone, furtherchilling the removed extract in a chilling zone so as to separate itinto a liquid solvent phase and a precipitated hydrocarbon phase and sothat the temperature of the liquid is maintained above the saturationtemperature of V the gas in said extract cooling zone and below thesaturation temperature of the gas in said first solvent heating zone,recovering the chilled solvent phase, a liquid hydrocarbon phase and asolid hydrocarbon phase from the chilled mixture, passing the chilledsolvent phase from said chilling zone to the aforesaid first solventheating zone for reheating by countercurrent contact with saidhumidified gas stream, returning the resulting relatively dry gas streamto the aforesaid extract cooling zone, passing the resulting reheatedsolvent to a second solvent heating zone and there countercurrentlycontacting it with the vapor-containing inert gas previously withdrawnfrom said extraction zone, returning the resulting reheated solvent fromsaid second solvent reheating zone to said extraction zone, and likewiserecycling the resulting s01- vent-denuded gas stream to said extractionzone.

15. A process according to claim 14 wherein the cyclic crystallinehydrocarbon phase consists essentially of durene and the inert gasisnitrogen.

16. A process according to claim 14 wherein the extract phase leavingthe extraction zone comprises 60 to 90 wt. percent ammonia, to 18 wt.percent monomethylamine, and 10 to 25 wt. percent hydrocarbons, andwherein the weight ratio of solvent to feed in the extraction zone isbetween 2.5 to 1 and 5 to 1.

17. A process for solvent recovery which comprises passing anammonia-hydrocarbon liquid extract through an extract cooling zone incountercurrent contact with a relatively dry inert gas whereby ammoniaevaporates into said gas and cools said liquid extract; passing theresulting humidified gas from said extract cooling zone into an ammoniaheating zone wherein said humidified gas is countercurrently contactedwith a chilled liquid ammonia phase; recycling the resultingdehumidified gas from said ammonia heating zone to said extract coolingzone in a plurality of streams so that the gas-to-liquid ratio is higherat the colder sections of said cooling and heating zones than at thewarmer sections thereof; removing the cooled liquid extract from saidextract cooling zone to a chilling zone wherein ammonia evaporates fromsaid liquid extract and thereby further cools said liquid extract to atemperature below the saturation temperature of the gas in said ammoniaheating zone; re covering the evaporated ammonia vapors; removing theresulting chilled slurry of a liquid ammonia phase, a liquid hydrocarbonphase and a solid hydrocarbon phase to a mechanical separation zone;recovering said solid phase from said chilled slurry; removing theremaining chilled mixture of liquid ammonia and liquid hydrocarbon to aphase separation zone; separating and recovering a liquid hydrocarbonphase and a liquid ammonia phase from said separation zone; andrecycling the chilled liquid ammonia phase to the aforesaid ammoniaheating zone wherein it is heated by the aforesaid contact with thehumidified gas; and recovering the heated liquid ammonia solvent.

18. A process for solvent recovery which comprises passing adurene-containing ammonia hydrocarbon liquid extract of a temperaturebetween about 40 and 90 F. in countercurrent contact with substantiallydry nitrogen gas through an extract cooling zone operating at a pressureof between about 150 and 400 p.s.i.a., whereby ammonia evaporates intosaid nitrogen cooling said liquid extract by about 60 to 100 F.; passingthe resulting humidified nitrogen from said extract cooling zone into anammonia heating zone operating at a pressure of between about 150 and400 p.s.i.a. wherein said humidified gas is countercurrently contactedwith a chilled liquid ammonia phase; recycling the resultingdehumidified nitrogen from said ammonia heating zone to said extractcooling zone in a plurality of streams so that the gas-to-liquid ratiois higher at the colder sections of said cooling and heating zones thanat the warmer sections thereof; removing the cooled liquid extract fromsaid extract cooling zone to a chilling zone wherein ammonia evaporatesfrom said liquid extract and thereby further cools said liquid extractto about -30 to 50 F. and to a temperature below the saturationtemperature of the gas in said ammonia heating zone; recovering theevaporated ammonia vapors; removing the resulting chilled slurry of aliquid ammonia phase, a liquid hydrocarbon phase, and a solidcrystalline durene phase to a mechanical separation zone wherefrom saidsolid durene phase is recovered as product from said chilled slurry;removing the remaining chilled mixture of liquid ammonia and liquidhydrocarbon to a phase separation zone; there separating a liquidhydrocarbon phase from a liquid ammonia solvent phase; recycling thechilled liquid ammonia phase to said ammonia heating zone wherein it isheated by about to F. by the aforesaid contact with the humidified gas;recovering the reheated liquid ammonia solvent; passing said liquidhydrocarbon phase from said separation zone to a scrubbing zone whereinsaid liquid hydrocarbon phase is contacted with water thereby removingremaining ammonia from said liquid hydrocarbon phase; separating theammonia-denuded liquid hydrocarbons from an ammoniacontaining waterphase in a separation zone; recovering said hydrocarbons; passing saidwater phase from said scrubbing zone to a distillation zone wherein theammonia is stripped from said water phase; recovering said ammonia; andrecycling the solvent-denuded water to said water-washing zone.

19. A process in accordance with claim 1 wherein said heating andcooling zones are maintained under substantially adiabatic conditions.

20. A process for extracting an ammonia soluble hydrocarbon from ahydrocarbon feed which comprises contacting said feed with warm liquidammonia in an extraction zone to form an extract, concomitantly passinginto said extraction zone a stream of inert gas at a temperature belowsaturation, evaporating a portion of the liquid ammonia into said inertgaseous st-ream thereby cooling the extract, removing the extract fromsaid extraction zone and separating cold liquid ammonia from the ammoniasoluble hydrocarbon components by further cooling, recovering theammonia soluble hydrocarbon components, passing a wet gas streamcontaining volatiles from said extraction zone to a solvent heating zonecountercurrent to the cold separated liquid ammonia thereby condensingvolatiles from said wet gas and warming said cold liquid ammonia,recovering warm liquid ammonia from said solvent heating zone for use insaid extraction zone.

References Cited in the file of this patent UNITED STATES PATENTSStephens Dec. 11,

14. A PROCESS WHICH COMPRISES EXTRACTING RELATIVELY SOLUBLE AROMATICCONSTITUENTS FROM A HYDROCARBON FEED IN A MULTISTAGE LIQUID EXTRACTIONZONE WHICH A LIQUID SOLVENT CONTAINING A SUBSTANTIAL FRACTION OF LIQUIDAMMONIA, PASSING A PLURALITY OF INERT GAS STREAMS THROUGH A PLURALITY OFSTAGES OF SAID LIQUID EXTRACTION ZONE SO AS TO EVAPORATE GRADUALLY OFSUFFICIENT PORTION OF THE LIQUID AMMONIA INTO SAID INERT GAS STREAMS TOCOOL SAID SOLVENT TO A TEMPERATURE 30* TO 50*F. COOLER THAN THEHYDROCARBON FEED STAGE, WITHDRAWING AN EXTRACT STREAM, PRECIPITATING APORTION OF THE HYDROCARBONS CONTAINED IN THE WITHDRAWN EXTRACT IN ASEPARATION ZONE, RECYCLING SAID PERCIPITATED HYDROCARBONS TO THEEXTRACTION ZONE COUNTERCURRENTLY TO AN REMAINING SOLVENT EXTRACT LAYERFROM SAID SEPARATION ZONE THROUGH AN EXTRACT COOLING ZONECOUNTERCURRENTLY TO ANOTHER RELATIVELY DRY INERT GAS STREAM WHEREBY APORTION OF VOLATILE SOLVENT EVAPORATES FROM SAID EXTRACT INTO THE GASSTREAM COOLING SAID EXTRACT, PASSING THE RESULTING HUMIDIFIED GAS STREAMFROM SAID EXTRACT COOLING ZONE INTO A FIRST SOLVENT HEATING ZONE, WHEREIT IS COUNTERCURRENTLY CONTACTED WITH A RELATIVELY COOL LIQUID SOLVENTPHASE, REMOVING THE COOLED EXTRACT FROM SAID COOLING ZONE, FURTHERCHILLING THE REMOVED EXTRACT IN A CHILLING ZONE SO AS TO SEPARATE ITINTO A LIQUID SOLVENT PHASE AND A PRECIPITATED HYDROCARBON PHASE AND SOTHAT THE TEMPERATURE OF THE LIQUID IS MAINTAINED ABOVE THE SATURATIONTEMPERATURE OF THE GAS IN SAID EXTRACT COOLING ZONE AND BELOW THESATURATION TEMPERATURE OF THE GAS IN SAID SOLVENT HEATING ZONE,RECOVERING THE CHILLED SOLVENT PHASE, A LIQUID HYDROCARBON PHASE AND ASOLID HYDROCARBON PHASE FROM THE CHILLED MIXTURE, PASSING THE CHILLEDSOLVENT PHASE FROM SAID CHILLING ZONE TO THE AFORESAID FIRST SOLVENTHEATING ZONE FOR REHEATING BY COUNTERCURRENT CONTACT WITH SAIDHUMIDIFIED GAS STREAM, RETURNING THE RESULTS RELATIVELY DRY GAS STREAMTO THE AFORESAID EXTRACT COOLING ZONE, PASSING THE RESULTING REHEATEDSOLVENT TO A SECOND SOLVENT HEATING ZONE AND THERE COUNTERCURRENTLYCONTACTING IT WITH THE VAPOR-CONTAINING INERT GAS PREVIOUSLY WITHDRAWNFROM SAID EXTRACTION ZONE, RETURNING THE RESULTING REHEATED SOLVENT FROMSAID SECOND SOLVENT REHEATING ZONE TO SAID EXTRACTION ZONE, AND LIKEWISERECYCLING THE RESULTING SOLVENT-DENUDED GAS STREAM TO SAID EXTRACTIONZONE.